Ixxv 



The tracheal cartilages present some difference in size and 

 form in different portions of the tube ; their diameter gradu- 

 ally, but slightly, increases from above downwards, and a 

 very trifling swell is observable in the region of the opening. 

 The upper and middle rings present flattened surfaces, and their 

 edges, above and below, are connected by elastic membrane; 

 those below the opening, though forming larger circles, are 

 small, round, and weak, and have an oblique direction down- 

 wards and forwards ; this portion of the tube is very elastic ; 

 the four or five last cartilages are deficient posteriorly, in a 

 narrow, angular interstice, which is closed by lining mem- 

 brane, weak muscular fibres, and elastic tissue ; from the last 

 ring no internal ridge or bar proceeds, but from the angle be- 

 tween the bronchi the mucous membrane rises mesially in a 

 prominent falx-like fold, which is increased or diminished in 

 depth and tension in proportion as the trachea and bronchial 

 tubes are shortened and contracted, or elongated and distended ; 

 the bronchial cartilages are little more than semicircles, their 

 extremities being somewhat thick and expanded ; the two su- 

 perior present, externally and posteriorly, small projecting 

 processes, into which some fibres from the long cervical mus- 

 cles are inserted ; the posterior wall of each bronchus is mus- 

 culo-membranous ; although there is no inferior larynx, yet 

 each bronchial opening, — which is of an oval form, the long 

 axis from before backwards, or, during life, from above down- 

 wards, — is capable of great alteration in size, figure, and ten- 

 sion of its lateral boundaries ; these changes can be effected 

 by the varying degrees of inspiration and expiration, by exten- 

 sion and flexion of the neck, and also by the action of the» 

 tracheal muscles. 



The six cartilages which are concerned in the tracheal 

 opening, deserve particular notice ; they are peculiarly elastic, 

 and appear composed, each, of two symmetrical portions, one 

 on either side; each lateral portion is crescentic (fig. 6); 

 the anterior cornu, flat, thin, and very moveable, is enveloped 



